The present invention relates to an electrostatic ink jet head that is used in an ink jet recording apparatus and causes liquid droplets containing dispersed particles to fly for image recording, and more particularly to an electrostatic ink jet head that allows ejection and flying of liquid droplets with stability.
Conventionally, an electrostatic ink jet recording system has been known which causes ejection of ink containing charged fine particle components by utilizing an electrostatic force generated through application of a predetermined voltage to each control electrode of an ink jet head based on image data, thereby recording an image corresponding to the image data on a recording medium. Various ink jet apparatuses using this electrostatic ink jet recording system are proposed (see JP 10-230608 A and JP 10-138494 A, for instance).
FIG. 3 is a schematic cross-sectional view schematically showing an ink jet head of a conventional ink jet recording apparatus disclosed in JP 10-230608 A.
As shown in FIG. 3, an ink jet head 100 has an insulating substrate 102 and a head substrate 104 which are arranged so as to oppose each other. A substrate through hole 102a is formed in the insulating substrate 102, and a control electrode 110 is provided around the substrate through hole 102a. Also, an ink guide 106 is provided at approximately the center of the substrate through hole 102a so as to stand on the head substrate 104. This ink guide 106 has a tip portion protruding from the substrate through hole 102a and includes an ink guide groove 108 formed by notching the ink guide 106 by a predetermined width along the center line of the ink guide 106.
Also, an ink reservoir 114 is formed between the insulating substrate 102 and the head substrate 104, and a signal voltage source 112 that supplies a signal voltage corresponding to an image that should be recorded is connected to the control electrode 110.
Further, a counter electrode 120 is provided so as to oppose a surface of the insulating substrate 102 on a protruding direction side of the tip portion of the ink guide 106. The counter electrode 120 is given a predetermined potential level and holds a recording medium P as a platen.
In addition, in the ink reservoir 114 in the ink jet head 100, an ink circulation mechanism (not shown) is provided which circulates ink Q through an ink supply tube (not shown) and an ink recovery tube (not shown).
It should be noted here that as the ink Q, ink is used in which charged colorant components (charged fine particles) is dispersed in a colloidal or suspended state in an insulating solvent having resistivity of 108 Ω·cm or more and is floated in the solvent.
In the ink jet head 100 having such a construction, the ink Q containing the colorant components moves upward in the ink guide groove 108 by capillary action and is gradually accumulated in the tip portion of the ink guide 106. When a high-voltage pulse is applied from the signal voltage source 112 to the control electrode 110 under this state, an ink droplet containing the colorant components flies out from the ink guide 106, is attracted by the counter electrode 120, and adheres onto the recording medium P. By ejecting multiple ink droplets in this manner, an image is recorded on the recording medium P.
FIG. 4 is a schematic cross-sectional view schematically showing an ink jet head of a conventional ink jet recording apparatus disclosed in JP 10-138494 A.
As shown in FIG. 4, in an ink jet head 130, an insulating support substrate 132 and a substrate 134 are arranged so as to oppose each other. An ink reservoir 136 is formed between the insulating support substrate 132 and the substrate 134. In the ink reservoir 136, an ink supply tank (not shown) is provided through a tube (not shown).
Also, a substrate through hole 132a is formed in the insulating support substrate 132. A first control electrode 140 and a second control electrode 142 are respectively formed on the front surface and the back surface of the insulating support substrate 132 so as to surround the periphery of the through hole 132a. Further, a metallic platen 120a is arranged so as to oppose a front surface side of the insulating support substrate 132. This metallic platen 120a doubles as a counter electrode and a recording medium P is held by the metallic platen 120a. 
In addition, a signal voltage source 144 is connected to the first control electrode 140 and the second control electrode 142, and a bias voltage source 146 is connected between the second control electrode 142 and the metallic platen 120a grounded. As the ink Q, conductive ink having conductivity of around 105 to 109 Ω·cm is used.
In the ink jet head 130 having such a construction, the ink Q is supplied from the ink supply tank to the ink reservoir 136, and the ink Q in the ink reservoir 136 is supplied into the through hole 132a by means of a hydrostatic pressure. Also, a bias voltage is applied to the first control electrode 140 and the second control electrode 142 by the bias voltage source 146. Under this state, a signal voltage based on an image signal is applied between the first control electrode 140 and the second control electrode 142 by the signal voltage source 144 so as to be superimposed on the bias voltage applied to the first control electrode 140 and the second control electrode 142. As a result, an ink droplet is caused to fly from an ink surface formed in the through hole 132a. This flying ink droplet is accelerated by the bias voltage applied to the metallic platen 120a as well as the first control electrode 140 and the second control electrode 142 and reaches the recording medium P. By ejecting multiple ink droplets in this manner, an image is formed on the recording medium P.
In the ink jet heads 100 and 130 shown in FIGS. 3 and 4, however, even at the time of ordinary usage, in particular due to influences of apparatus vibrations, ink supply pressure fluctuations, and the like, a phenomenon such as ink seepage, changing of the shape of an ink meniscus formed at each tip of ink guide, or changing of the positional relationship between the meniscus and the tip of ink guide occurs and influences print characteristics. In particular, in the case of a line head and the like where it is required to arrange ink ejection portions at high density, there occurs a problem in that due to interference of ink between adjacent ejection portions, it becomes impossible to control the diameter of each ink droplet, making it difficult to record an image of high quality.